A bicycle is normally provided with a rear derailleur active on a sprocket assembly, which consists of a series of coaxial toothed wheels (sprockets) having different diameters and numbers of teeth coupled with the hub of the rear wheel.
A bicycle can furthermore be provided with a front derailleur associated with a crankset, which consists of a series of toothed wheels (crown gears) having different diameters and number of teeth associated with a pin of the bottom bracket driven in rotation by crank arms.
Usually, there are two or three crown gears of the crankset having increasing diameters starting from the crown gear closest to the bicycle frame.
The derailleur, whether front or rear, engages a transmission chain extending in a closed loop between the sprocket assembly and the crankset, moving it on toothed wheels having a different diameter and number of teeth, so as to obtain different gear ratios.
In particular, downward gearshifting is when the chain passes from a toothed wheel having a larger diameter to a toothed wheel having a smaller diameter, and upward gearshifting is when the chain moves from a toothed wheel having a smaller diameter to a toothed wheel having a larger diameter. Concerning this, it should be noted that with reference to the front derailleur, downward gearshifting corresponds to the passage to a lower gear ratio and upward gearshifting corresponds to the passage to a greater gear ratio.
The movement in the two directions of the front derailleur is obtained through an actuation device mounted on the handlebars so as to be easy for the cyclist to maneuver.
More specifically, in a mechanical gearshift, the front derailleur is moved between the crown gears of the crankset, in a first (upwards) direction by a traction action exerted by a normally sheathed inextensible control cable (commonly called Bowden cable), in a second opposite (downward) direction by the release of the traction of the cable and by the elastic return action of a spring provided in the derailleur itself.
The front derailleur is actuated by a linkage that actuates a chain guide adapted for physically moving the chain between different engagement positions on the crown gears.
The chain guide comprises an inner plate and an outer plate substantially parallel to one another and substantially parallel to the planes of the crown gears. The inner plate is spaced from the outer plate and both are simultaneously and rigidly translated by the traction or by the release of the control cable.
During downward gearshifting, the outer plate of the derailleur intercepts the chain and derails it from the crown gear engaged, making it fall onto a smaller crown gear arranged closer to the frame.
During upward gearshifting, the inner plate of the derailleur intercepts the chain and derails it, accompanying it, towards a larger crown gear arranged further from the frame.
In the actuation device, the control cable is actuated in traction or in release through winding and unwinding on a rotor element, commonly called cable-winding bush, the rotation of which is actuated by the cyclist with a suitable control lever, or with two control levers (a first lever for upward gearshifting and a second lever for downward gearshifting).
In any case, the actuation device must foresee that the cable-winding bush is held in a number of predetermined angular positions, angularly spaced apart by predetermined indexing angles between a first and a last indexing position. This function is obtained with so-called indexers, many types of which are known in the field, variously active between the cable-winding bush and the fixed casing of the device.
In indexers, there are usually two, and sometimes even three, indexing positions when the transmission chain is positioned on the smallest crown gear of the crankset. This is to ensure correct positioning of the derailleur irrespective of the toothed wheel of the sprocket assembly engaged by the transmission chain.
For the same reason, there are usually two indexing positions when the transmission chain is positioned on the largest crown gear of the crankset.
A pair of mechanical end stops, and in particular an inner end stop and an outer end stop, limit the excursion of the derailleur to prevent it from derailing the chain too much towards the frame or too far away from the frame (thus ensuring that the chain does not derail beyond the crown gears).
Although the derailleur has a predetermined geometry, its effective excursion following the actuation of the actuation device is a function of the effective mounting configuration of the derailleur and of the actuation device, said mounting configuration, even if actuated within predetermined mounting tolerances, varying from case to case. For example, the effective force (or tension) transmitted by the control cable following the same rotation of the cable-winding bush (also) depends on the particular path followed by the control cable to reach the derailleur, on the way in which the control cable has been connected to the derailleur and on many other factors linked to the installation of the derailleur and of the control device on the bicycle.
Clearly, such variations of the effective excursion of the derailleur are small in size, but they are sufficient in size that it is not possible to ensure that the total excursion of the derailleur (useful for carrying out all gearshifting operations) corresponds exactly to the angular excursion of the cable-winding bush between the first and the last indexing position.
The indexing positions and the relative controls are therefore sized so as the ensure correct gearshifting also in the most unfavorable mounting situation, implying that the angular distance between the first and the last indexing position corresponds to a useful stroke of the derailleur that is greater than what is actually necessary.
For this reason, the mechanical end stops of the derailleur are usually adjusted so that the outer end stop acts as a stop for the excursion of the derailleur when the cable-winding bush is held in the last indexing position, in other words in the indexing position reached after the last upward gearshifting.
In this way, it is possible to ensure for the cyclist that the last upward gearshifting operation is obtained and stably maintained.
The inner end stop is adjusted to ensure that the derailleur is positioned on the smallest toothed wheel of the crankset avoiding sliding of the transmission chain on the derailleur when the transmission chain is engaged on the largest toothed wheel of the rear cassette.
In this way, according to the above, the excursion of the derailleur is stopped before the cable-winding bush reaches the first indexing position. However, the action exerted by the cyclist on the control in any case carries the cable-winding bush into the first indexing position, making the gearshifting stable. Indeed, in this situation (downward gearshifting) the rotation of the cable-winding bush towards the first indexing position causes a loosening of the control cable that thus does not prevent the rotation of the cable-winding bush.
The Applicant has noted that the adjustment of the mechanical end stops of the aforementioned derailleur results in a lower responsiveness at least in the first upward gearshifting. This is since the first part of the first upward gearshifting does not cause any movement of the derailleur and has the sole purpose of recovering the tension of the control cable.
The Applicant has perceived that by adjusting the inner mechanical end stop exactly at the first indexing position there would be an immediate and rapid response in the first upward gearshifting operation.
The Applicant has however found that in this way the outer mechanical end stop should be adjusted so as to interrupt the rotation of the cable-winding bush before it reaches the last indexing position. The reaching of the stable position of the cable-winding bush on the last indexing position would be strongly counteracted by the fact that the control cable should be wound further on the cable-winding bush with the derailleur impeded by the outer end stop to carry out a further translation.